1. Field of the Invention
The present invention relates to an optical film made from a homo polypropylene resin and particularly, to a retardation film having a zero wavelength dispersibiliy and a uniform in-plane retardation value, and a laminated optical film having a positive wavelength dispersibility. More particularly, the present invention relates to a retardation film having a zero wavelength dispersibility having a uniform in-plane retardation value and enhanced durability, and a laminated optical film having positive wavelength dispersibility which substantially matches theoretical broadband values in the whole wavelength of a visible region.
2. Description of the Related Art
A retardation film is made from a material of various polymerized or copolymerized polymeric resins according to target optical characteristics.
A polypropylene resin is typically categorized, based on types of copolymer polymerized therewith, into a homo polymer, a random copolymer, and a block copolymer. The polypropylene resin is classified as an isotactic resin, a syndiotactic resin or an atactic resin, based on stereo regularity of molecular chains in the polypropylene resin. In the conventional technology of manufacturing optically anisotropic films by using the polypropylene resin, a random copolymer and a block copolymer being amorphous or low crystalline that has no stereo regularity or has a very low stereo regularity, such as an ethylene-propylene copolymer resin, is widely used because control of a transparent characteristic and an optical anisotropic characteristic as resin material is relatively easy.
However, when the ethylene-propylene copolymer resin is used as a resin material, a mechanical property, such as stiffness or tensile strength, are low because of amorphousness or low crystallinity of the copolymer resin. Accordingly, when the ethylene-propylene copolymer resin is applied to a manufacturing process, the manufactured film becomes thick, and thus the manufactured film has a low transparency. Furthermore, since the ethylene-propylene copolymer resin has a low melting point, the heat resistance of the manufactured film is inferior when the film is used at a high temperature or is continuously used. In addition, the ethylene-propylene copolymer resin has a low molecular weight in comparison with the homo polypropylene resin of a propylene homo polymer. Thus, the manufactured film has a high melt flow rate (MFR). In addition, the manufactured film fails to have a desirable optical anisotropy because molecular orientation by mechanical stretching is not easy (e.g., the film is broken when a stretching ratio (draw ratio) gets high or stretching (drawing) speed gets fast). For these reasons, commercialization is difficult because it is not suitable for mass production and high-speed production. As a result, a broadband retardation film is required that can have a predetermined retardation value over the whole wavelength of a visible region (400 nm to 800 nm), when the film is used for reflective or transflective liquid crystal display devices suitable for new uses, such as display devices for mobile and outdoor advertisements, and optical pick-up parts for an optical disk. For instance, to exhibit a broadband retardation having a uniform retardation value (e.g., λ/4) for transmitted light, an optical film is required in which the retardation value increases as the wavelength of the transmitted light increases.